1. Field of the Invention
The invention relates to the field of magneto-resistive heads, and in particular, to circuitry for correcting asymmetry in the output (read signal) from these heads.
2. Background Information
The magneto-resistive heads currently used in disk-drives sometimes produce asymmetric pulses due to various reasons which include temperature and bias point variation.
Asymmetry of a waveform such as the one shown in FIG. 2 is given by the equation:
Asymmetry=(MAXxe2x88x92MIN)/MAX
So for example if MIN is 30 mV and MAX is 60 mV (absolute value), the asymmetry would be 50%. This would be defined as negative asymmetry (shown in FIG. 2).
The problem of asymmetry has been solved before using circuit techniques. The purpose of a correction circuit is to take such an asymmetric waveform and shape it such that the positive and negative pulses are equal (i.e. symmetrical).
U.S. Pat. No. 6,072,647, shows a method of correcting MR Head Asymmetry by changing the bias current in the head. This is based on a reproduced signal.
U.S. Pat. No. 6,043,943 (also Japanese Patent No. 10-214403) shows a method of asymmetry correction which requires matching delay lines.
U.S. Pat. No. 5,943,177, shows a method of defection of asymmetry and a method of correction based on peak detection and rectifier circuits. Also, the method includes separating the positive and the negative lobes of the signal.
JAPANESE Patent No. JP623410, shows a method of correction which changes the bias current in the MR head.
U.S. Pat. No. 6,052,245, describes a detection method which is a delay-based, digital implementation of the squaring method of correction of asymmetry.
U.S. Pat. No. 5,999,349, describes a digital way of correcting asymmetry. This patent describes a method in which tap weights are changed to change the waveform shape. This method of correction of asymmetry is complicated.
U.S. Pat. No. 5,986,831, deals with MR asymmetry detection which uses a digital filter to differentiate samples into states and then uses circuits to create an equalized waveform.
U.S. Pat. No. 5,787,005, describes a method based on peak-hold circuits.
Many of the prior asymmetry correction schemes depend on changing head bias current or using matching delay lines for their correction to work, so the performance is very much dependent on layout and process parameters, which are hard to control.
Therefore, a need exists for an improved asymmetry correction circuit which is independent of layout and process parameters which are hard to control, and therefore, overcomes the problems and limitations of the prior art.
It is, therefore, a principle object of this invention to provide a magneto-resistive asymmetry correction circuit.
It is another object of the invention to provide a magneto-resistive asymmetry correction circuit that solves the above mentioned problems so that asymmetric pulses from magneto-resistive heads are corrected simply and effectively.
These and other objects of the present invention are accomplished by the method and apparatus disclosed herein.
According to an aspect of the invention, an implementation of an exemplary embodiment is well-suited for a read channel in BICMOS technology because of its range of correction and the size of the actual circuits.
According to an aspect of the invention, an exemplary embodiment has been shown to be effective in both laboratory and simulation.
According to an aspect of the invention, an exemplary embodiment advantageously increases magneto-resistive (MR) head yield since placing the circuit in the read path tolerates larger head asymmetries and is capable of obtaining acceptable error rates on drives. By being able to ship heads with larger asymmetry, component yield can be increased.
According to an aspect of the invention, correcting asymmetry is advantageously achieved, not just detection or measurement of an amount of asymmetry.
According to an aspect of the invention, an exemplary embodiment of the present invention advantageously does not involve changing biasing conditions in the magneto-resistive (MR) head. Instead, the exemplary embodiment uses a novel way of analog wave shaping by employing non-linear and linear differential stages in the read channel.
According to an aspect of the invention, an exemplary embodiment corrects asymmetry such that the positive and negative pulses are equal.
According to an aspect of the invention, an exemplary embodiment corrects asymmetry in a very different way than has previously been done. In contrast to known asymmetry correction circuits, which depend on matching delay lines for their correction to work, and exemplary embodiment of the present invention does not require matching delay lines. Therefore, the performance of the exemplary embodiment of the present invention is independent of layout and process parameters which are hard to control.
According to an aspect of the invention, an arrangement for correcting magneto-resistive head asymmetry, includes a shift circuit which receives a magneto-resistive head read signal, and a polarity signal indicative of the polarity of any asymmetry of the read signal. Depending on the polarity of the asymmetry, the shift circuit either adds or subtracts a shift value to the read signal to produce a shifted read signal. The shift circuit outputs the read signal, the shifted read signal, and the shift value. First, second and third gain circuits, receive the read signal, the shifted read signal, and the shift value, respectively, and each receive a respective control signal. The first, second and third gain circuits provide respective outputs amplified in accordance with the respective control signals.
According to an aspect of the invention, the respective control signals and the polarity signal are derived from the read signal by control means, the control means receiving the read signal, detecting a polarity of any asymmetry of the read signal, and providing the polarity signal to the shift circuit and the respective control signals to the first, second and third gain circuits, based on an amount of correction required.
According to an aspect of the invention, a common mode adjustment means is provided for receiving the respective outputs from the first, second and third gain circuits, as a combined output, and for correcting any common mode effect in the combined output. The common mode adjustment means includes a common mode feedback circuit. A pair of emitter followers is provided for receiving the respective outputs from the first, second and third gain circuits, and providing the combined output to the common mode adjustment means.
According to an aspect of the invention, the control means includes detection means for receiving the read signal, detecting any asymmetry in the received read signal and the polarity of any asymmetry, producing the polarity signal, and producing a digital correction signal indicative of an amount of asymmetry correction required, and a digital to analog converter for receiving the digital correction signal and producing the respective control signals to the first, second and third gain circuits, based on an amount of correction required.
According to an aspect of the invention, the first, second and third gain circuits include differential amplifier pairs having certain transconductance characteristics which can be changed by adding current to one side of the differential pair. These amplifier pairs are differential bipolar transistor pairs in one embodiment.
According to an aspect of the invention, a method of correcting asymmetry in a read signal from a magneto-resistive head, includes detecting an amount of asymmetry in the read signal, and the polarity of the asymmetry detected, and producing a corrected read signal by combining the outputs of first second and third gain stages. The first gain stage is a linear gain stage which receives the read signal unaltered, the second gain stage is a non-linear gain stage which receives the read signal altered by a shift value in accordance with the polarity of the asymmetry detected, and the third gain stage receives the shift value. The gain stages have a respective gain thereof controlled based on the asymmetry detected.
These and other aspects of the invention will become apparent from the detailed description set forth below.